if your aerodynamics is like that of the cavalier, then about 90% of your cooling comes from underneath. on my car there is a flap that attaches to the bottom of the radiator that directs air towards the radiator while going down the road. I know the old 90's camaro was the same way. if the flap gets knocked off, you start to run hot.
I have my entire lower grill blocked off. I used to have the opening between my hood and bumper blocked off all but enough room to pop the hood up. I was just lazy on that and didn't put it back on.
I have had my lower grill blocked off for over 3 years. about 1.5 to 2 years ago I permanently sealed it off. I will admit that my body work skills are horrible but it got the job done and looked good for a little while
I would say to research the crap out of your vehicle before doing it but many of us do have complete grill blocks and though I don't see many 100 degree days, we have seen a record number of upper 90 degree days this year. no problems yet. 14 yrs old and 218k miles (7 yrs and about 125k are from me personally).
I started with plexi glass too. made a paper template and used a dremel to cut the plexi. put it on with wing nuts, bolts, and fender washers.
i'm unsure of how to upload pics onto here? but i came across a shelby cobra carbon fiber front air splitter and i'm going to apply that to my car soon. i'm still getting the parts together for my exhaust.